This invention relates to a non-aqueous electrolyte battery that controls vaporization and decomposition of the electrolyte using an organic solvent as a base material and provides excellent cell performance. More specifically the invention relates to the non-aqueous electrolyte battery that has excellent cell performance, high-temperature preservability and reduced danger of bursting and ignition even in the case of short-circuiting.
Recently non-aqueous electrolyte secondary batteries have drawn public attention as batteries for backing up information-processing equipment such as personal computers, VTRs, etc. and memories for AV (audio visual) equipment as well as for driving these devices because they have superior self-discharge characteristics such as high-voltage output and high energy density.
Although the non-aqueous electrolyte secondary battery exhibits such high performance as mentioned above, it has certain disadvantages. For example, in the lithium battery which uses pure lithium metal for the negative electrode thereof, alkaline metal especially lithium metal, lithium alloys, etc. that are used in the negative electrode of the above-mentioned battery are highly active to water. When water is intruded into the battery because of incomplete sealing of the sealed entrance thereof, there is a fear of generating hydrogen and occurrence of ignition due to the reaction between the above-mentioned negative electrode and water. Moreover, since lithium metal has a low melting point of approximately 170xc2x0 C., there is always a possibility that the battery goes into abnormal heat generation and hence runs into an excessively dangerous condition such as melting of the battery itself or the like if the battery discharges a large amount of current abruptly in the case of short-circuiting. In addition, if the electrolyte whose base material is an organic solvent evaporates or decomposes by itself to produce a gas with the heat generation of the battery or if the battery is misused, the gas may cause bursting and ignition.
As method for avoiding danger of bursting and ignition of the battery in the case of short-circuiting, there have been proposed many methods, for example, in the following: (a) a method (Japanese Unexamined Patent Application Publication No. 01-14877) where a non-aqueous electrolyte is used that is made by dissolving LiCF3 SO3 as a solute in an electrolyte composed of a mixture of sulfolane and one kind of organic solvent selected from the group consisting of dimethoxyethane, tetrahydrofuran, diethoxyethene, 2-methyldioxolane, 4-methyldioxolane, and dioxolane; (b) a method (Japanese Unexamined Patent Application Publication No. 01-14878) where an electrolyte is used that is made by dissolving LiCF3 SO3 as a solute in either organic solvent of N, N-dimethylformamide or N, N-diethylamide; (c) a method (Japanese Unexamined Patent Application Publication No. 01-14879) where an electrolyte is used that is made by dissolving at least one substance selected from the group consisting of LiPF6 and LiBF4 as a solute in an electrolyte made by mixing sulfolane and at least one kind of organic solvent selected from the group consisting of dimethoxyethane, 2-methyldioxolane, 4-methyldioxolane, and 4-tetrafuran; (d) a method (Japanese Unexamined Patent Application Publication No. 01-14880) where an electrolyte is used that is made by dissolving at least one substance selected from the group consisting of LiCF3 SO3, LiPF6, LiAsF6, LiClO4 in a mixture of a high-permittivity organic solvent whose permittivity is 30 or more at 20xc2x0 C. and a low-viscosity organic solvent whose inflammation point is 30xc2x0 C. or more; (e) a method (Japanese Unexamined Patent Application Publication No. 02-165565) where oligoalkylene oxypolyphosphazen having a fluoroalkylsulfon group is used as an electrolyte; (f) a method (the Japanese Unexamined Patent Application Publication No. 04-184870) where a lower phosphate ester is used as a solution of the electrolyte or a co-solvent; (g) a method (Japanese Unexamined Patent Application Publication No. 06-283205) where a flame retarder such as perfluorocarbon that is a fluorine compound inert liquid, phosphide, etc. is made to be contained in an electrolytic solution. On the other hand, as a non-aqueous electrolyte to be used for the lithium battery and the lithium ion secondary battery, widely used is a substance made by dissolving LiPF6 as an additional electrolyte in a carbonate system electrolyte such as propylene carbonate, diethyl carbonate, etc. because it has a relatively high conductivity and is stable in the electric potential. Among the batteries which use these non-aqueous electrolytes, the lithium ion secondary battery is well known to have higher safety compared to batteries which use lithium metal. These carbonate system non-aqueous solvents are said to be electrochemically relatively stable, but since in the non-aqueous electrolyte battery, oxidizability and reducing power of the positive and negative electrodes are exceedingly strong; therefore there is a possibility that these carbonate system non-aqueous solvents may react with the positive electrode active materials. If such a reaction occur, a reaction products and decomposition products grow on the surface of the electrode as a film, which increases the impedance of the battery. As a result, there is a problem that when the battery is discharged with a specially large current, the voltage decreases significantly to deteriorate cycling and load characteristics.
For a method for controlling decomposition of the non-aqueous electrolyte, proposed are a method (J. L. Goldman, R. M. Mank. J. H. Young and V. R. Koch: J. Electrochem. Soc., 127,1461(1980)) where some of hydrogen atoms of a cyclic ether such as tetrahydrofuran, 1,3-dioxolane, etc. are substituted with an alkyl group to stabilize it by transforming into 2-methyltetrahydrofuran, 4-methyl-1,3-dioxolane, etc. and a method (Japanese Unexamined Patent Application Publication No. 07-320779) whereby a sulfide compound such as methylphenyl sulfide, diphenyl sulfide, thianthrene, 1,8- disulfide naphthalene, etc. is added to the non-aqueous electrolyte.
Further, in the Japanese Patent Application Unexamined Publication No. 06-13108, disclosed is a fact that by using a solution such that a lithium salt is dissolved in a phosphazene derivative having a viscosity less than 300 cP at 25xc2x0 C. as an electrolyte for a non-aqueous electrolyte secondary battery, safe and excellent cell performance was given to the battery. The phosphazene derivatives disclosed here are a cyclophosphazene which is expressed by (NPR2 )n, where R denotes a monovalent organic group and n is a number of 3 to 15 and a linear phosphazene derivative which is expressed by R3 (Pxe2x95x90N)mxe2x80x94PR4, where m denotes a number of 1 to 20, and R is selected from the group consisting of monovalent organic groups, O, and C.
However, a method as mentioned above wherein a kind of a chemical is added to the conventional electrolyte can be effective only in the range of ordinary temperature but has a disadvantage, from the point of view of the decomposition of the electrolyte, that the decomposition of the electrolyte still continues at a high temperature, and hence the cycling characteristic deteriorates. From the point of view of flameproofing of the electrolyte, the method has a disadvantage that although the flame resistance is improved to some degree, at the same time the cell performance deteriorates. Moreover, also regarding the phosphazene derivative, a compound that can give much more excellent cell performance has been desired to come into being.
Therefore, it is an object of the present invention to provide a non-aqueous electrolyte battery that is capable of controlling evaporation and decomposition of the electrolyte whose base material is an organic solvent throughout a wide range of temperatures, reducing danger of bursting and ignition when short circuiting, and excelling especially in high-temperature preservability, resulting in superior cell performance.
Under such conditions of the current technology, the present inventors carried out intensive examination for the non-aqueous electrolyte battery and discovered that if a specified phosphazene derivative having a sulfonyl group was made to be contained in the electrolyte of the non-aqueous electrolyte battery, the phosphazene derivative so contained was able to prevent evaporation and decomposition of the electrolyte throughout a wide rage of temperatures, especially at a high-temperature, and thereby can give flame resistance to the non-aqueous electrolyte, reduce the danger of the ignition and inflammation. At the same time, excellent cell performance can be obtained, leading to the present invention.
That is, the present invention provides a non-aqueous electrolyte battery including a positive electrode, a negative electrode capable of occluding and emitting lithium ions, and a non-aqueous electrolyte containing lithium ion, characterized in that the non-aqueous electrolyte is a solution containing at least one kind of the phosphazene derivatives selected from the group consisting of the phosphazene derivatives expressed by a general formula (1):
(R1O)3Pxe2x95x90Nxe2x80x94SO3R1xe2x80x83xe2x80x83(1)
where R1 denotes a same or different monovalent organic group in the expression and phosphazene derivatives expressed by a general formula (2):
(R2O)3Pxe2x95x90Nxe2x80x94SO2xe2x80x94Nxe2x95x90P(OR2)3xe2x80x83xe2x80x83(2)
where R2 denotes a same or different monovalent organic group, and a lithium salt.